“Nanocrystalline” or “nanosized” particles are hereinafter to be understood to mean particles, the average size of which is not more than 100 nanometers (nm) in diameter and particularly preferably not more than 30 nm in diameter. “Organo-metallic precursors” are used as carrier media, and which can include dispersion aids for ensuring that the particles are uniformly dispersed in a medium.
“Nanocrystalline composites”, in the present invention, refer predominantly to bulk materials or coatings which contain at least one phase of materials and in which the grains of all phases are less than 100 nm, either in starting materials known as “green bodies” or in final products such as coatings, ceramics or composites.
The first step in the production of a nanocrystalline particle or nanoparticle to a fully dense composite is the compaction of powder comprising of nanocrystalline particles to form a “green body”, which is an intermediate product. This is conventionally done by using an uni-axial press.
Decades of research on conventional ceramic processing have shown that the uniformity and density of particle packing in the green body has a significant influence on how well the green body will densify and consequently on how large the sizes of grains will develop to during sintering. Conventional nanocrystalline particles, however, are at a disadvantage in this regard.
There are essentially two problems encountered, namely the agglomeration of particles in the processing of these nanocrystalline particles and the difficulty of producing of ceramic green bodies with high solid content or high green density.
The first problem, agglomeration of particles in the nanometer range, is common because of inter-particle London-van der Waals forces and functional groups on the particle surface. Most nanocrystalline particles currently made are not composed simply of individual single crystals or nanometer-sized particles, known as “crystallites”. Crystallites made by conventional methods tend to bond and form larger units termed “agglomerates” or “aggregates”.
Thus, the true particle size in most powders is the agglomerate or aggregate size. The presence of either agglomerates or aggregates is extremely deleterious during the sintering of nanocrystalline particles into a solid with grain sizes less than 100 nm. As a rule of thumb, in pressureless sintering of the prior art, it is difficult to obtain a grain size which is less than the starting agglomerate size. Indeed, early attempts at pressureless sintering nanocrystalline ceramics to full density never succeeded in producing less than 100 nm grain sizes. Typically, grain sizes in the order of a micrometer were produced. In large part, this was because samples were fabricated from agglomerated powders. A disadvantage of the prior art is the agglomeration of nanocrystalline particles.
Another problem in the prior art is the low density of green bodies due to high porosity within the green bodies. In conventional methods, nanocrystalline particles are compressed to form “powder compacts”. These powder compacts, before they are heated, are composed of individual crystallite or agglomerate separated by between 25 and 60 vol % of porosity, depending on the particular material and processing method used. To improve properties such as strength, translucency and so on, in the final product, it is desirable to reduce as much of this porosity as possible. It is evident that the more homogeneous the particle packing is and the higher solid content a green body has, the easier it will be to get a uniform and dense microstructure during sintering. However these are hard to achieve with conventional techniques.
Nanocrystalline particles, however, have poor compaction behavior compared to conventional sub-micron particles. This can, in many cases, be attributed to the agglomeration present in the starting powders. The compaction of fractal-like agglomerates produces an inhomogeneous particle packing structure within the green body. Even if the agglomerates themselves are extremely dense, large inter-agglomerate pores may lead to poor density in the green body. There are also other difficulties with the compaction of nanocrystalline particles which cannot be attributed to agglomerations.
One known difficulty is the large number of particle-particle point contacts per unit volume in nanocrystalline particles. Each of these point contacts represents a source of frictional resistance to the compaction of the powder and this inhibits particle-particle sliding and particle rearrangement. It is therefore desirable to provide a method with which the nanocrystalline particles can slide against one another and rearrange themselves easily so as to produce a more homogeneously packed green body with relatively high density.
Another problem in conventional methods is the inability to retain an ultra-fine grain size in the product after sintering. Major obstacles include the strong tendency of nanocrystalline particles to agglomerate and the ever-present obstacle of unwanted grain growth during sintering. This is because the capillary driving force for sintering (involving surfaces) and grain growth (involving grain boundaries) are comparable in magnitude, both being proportional to the reciprocal grain size. This means that the final-stage sintering processes are inevitably accompanied by the rapid grain growth.
Thus it is now generally recognized that unless this grain growth problem can be overcome, the conventional pressureless sintering process cannot produce dense ceramics with nanometer-scale structure (grain size less than 100 nm), leading many researchers to resort to the approach of high-pressure consolidation. High pressure results in a marked increase in the number of nucleation events so that the final grain size is small. This is because final grain size is determined by the number of nuclei formed, that is, the number of grains present that can impinge on one another. It is therefore desirable to provide a method to promote the nucleation of primary crystallization and control its uniformity during sintering.
Most recently, a simple two-step sintering method without applied pressure was reported by Chen & Wang (Nature 404:168-171; 9 Mar. 2000) where fully dense cubic Y2O3 with a grain size of 60 nm was successfully prepared. That sintering method used a two-step heating schedule. The sample was first heated to a high temperature to achieve an intermediate density, then cooled and held at a lower temperature until it was fully dense. To succeed such a two-step sintering, a sufficient high starting density should be obtained during the first step. When the density is above 70% porosity, data have shown that all pores in Y2O3 become sub-critical and unstable against shrinkage (which occurs by capillary action). These pores can be filled as long as grain boundary diffusion allows it, even if the particle network is frozen as it clearly is in the second step. It was found that densities higher than 75% were adequate for subsequent second step sintering. This result further emphasizes the importance of the production of green bodies with high solid content in nanocrystalline composite preparation.
Having noted the abovementioned problems of agglomeration and poor uniformity and density, Helmut Schmidt et al, in U.S. Pat. No. 5,590,387, disclosed a method to overcome these problems by modifying nanocrystalline particles with surface modifier and then re-dispersing it in water, an organic solvent or a mixture of both, to form a suspension. However, this improvement itself presented a disadvantage as the surface modifier and organic solvent were entirely removed after burning out, the green body ready for sintering still did not have sufficiently high density.
The present invention seeks to overcome or at least alleviate these problems in the prior art.